Conventionally, in order to perform communication between specific parties, there has been adopted a configuration in which original in formation (key information) for encoding/decoding is shared between transmitting and receiving ends, and secret communication is realized by performing a mathematical operation/inverse operation on information data (plain text) to be transmitted, in accordance with the information. FIG. 20 is a block diagram showing a configuration of a conventional data transmitting apparatus based on the configuration. As shown in FIG. 20, the conventional data communication apparatus includes a data transmitting apparatus 90001, a transmission line 913, and a data receiving apparatus 90002. The data transmitting apparatus 90001 is composed of an encoding section 911 and a modulation section 912. The data receiving apparatus 90002 is composed of a demodulation section 914 and a decoding section 915. When information data 90 and first key information 91 are inputted to the encoding section 911, and when second key information 96 is inputted to the decoding section 915, information data 98 is outputted from the decoding section 915. In order to describe eavesdropping by a third party, it is assumed that FIG. 20 includes an eavesdropper data receiving apparatus 90003 which is composed of an eavesdropper demodulation section 916 and an eavesdropper decoding section 917. Third key information 99 is inputted to the eavesdropper decoding section 917. Hereinafter, with reference to FIG. 20, an operation of the conventional data communication apparatus will be described.
In the data transmitting apparatus 90001, the encoding section 911 encodes (encrypts) the information data 90 by using first key information 91. The modulation section 912 modulates the information data, which is encoded by the encoding section 911, into a modulated signal 94 in a predetermined modulation format so as to be transmitted to the transmission line 913. In the data receiving apparatus 90002, the demodulation section 914 demodulates, in a predetermined demodulation method, the modulated signal 94 transmitted via the transmission line 913, and outputs the encoded information data. The decoding section 915 decodes (decrypts) the encoded information data by using the second key information 96, which is shared with the encoding section 911 and is identical to the first key information 91, and then outputs original information data 98.
When the eavesdropper data receiving apparatus 90003 eavesdrops a modulated signal (information data) which is transmitted between the data transmitting apparatus 90001 and the data receiving apparatus 90002, the eavesdropper demodulation section 916 causes a part of the modulated signal transmitted through the transmission line 913 to be divided, to be inputted thereto, and to foe demodulated in the predetermined demodulation method. The eavesdropper decoding section 917, thereafter, attempts to decode the same by using third key information 99. The eavesdropper decoding section 917 does not share key information with the encoding section 911. That is, the eavesdropper decoding section 917 performs decoding by using the third key information 99 which is different from the first key information 91, and thus cannot reproduce the original information data appropriately.
A mathematical encryption (or also referred to as a computational encryption or a software encryption) technique based on such a mathematical operation may be applied to an access system or the like, as described in publication of patent document 1, for example. In ether words, in the case of a PON (Passive Optical Network) configuration in which an optical signal transmitted from one optical transmitter is divided by an optical coupler so as to be distributed to optical receivers at a plurality of optical subscribers' households, the optical signal only desired by and supposed to foe directed to certain subscribers is inputted to all the optical receivers. Therefore, information data for respective subscribers is encoded by using key information which is different depending on the subscribers, whereby leakage/eavesdropping of mutual information may be prevented, and safe data communication may be realised.
Patent document 1: Japanese Laid-Open Patent Publication No. 9-205420
Non-patent document 1: “Cryptography and Network Security: Principles and Practice” translated by Keiichiro Ishibashi et al., Pearson Education, 2001
Non-patent document 2: “Applied Cryptography” translated by Mayumi Adachi et al., Softbank publishing, 2003